Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as a power source for electric vehicles (EV) and hybrid electric vehicles (HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel.
Small-sized mobile devices use one or several battery cells for each device. On the other hand, middle- or large-sized devices, such as vehicles, use a middle- or large-sized battery module having a plurality of battery cells electrically connected with each other because high power and large capacity are necessary for the middle- or large-sized devices.
Preferably, the middle- or large-sized battery module is manufactured with small size and weight if possible. For this reason, a prismatic battery or a pouch-shaped battery, which can be stacked with high integration and has a small weight to capacity ratio, is usually used as a battery cell of the middle- or large-sized battery module. Especially, much interest is currently generated in the pouch-shaped battery, which uses an aluminum laminate sheet as a sheathing member, because the weight of the pouch-shaped battery is small, the manufacturing costs of the pouch-shaped battery are low, and it is easy to modify the shape of the pouch-shaped battery.
For the middle- or large-sized battery module to provide power and capacity required by a predetermined apparatus or device, it is necessary for the middle- or large-sized battery module to be constructed in a structure in which a plurality of battery cells are electrically connected in series with each other, and the battery cells are stable against an external force.
Also, the battery cells constituting the middle- or large-sized battery module are secondary batteries which can be charged and discharged. Consequently, a large amount of heat is generated from the high-power, large-capacity secondary battery during the charge and discharge of the battery cells. If the heat, generated from the unit cells during the charge and discharge of the unit cells, is not effectively removed, the heat accumulates in the respective unit cells, and therefore, the deterioration of the unit cells is accelerated. According to circumstances, the unit cells may catch fire or explode. For this reason, a cooling system is needed in a battery pack for vehicles, which is a high-power, large-capacity battery.
An example of the battery pack cooling system is illustrated in FIG. 1.
Referring to FIG. 1, the battery pack cooling system includes a battery module 20 constructed in a structure in which a plurality of battery cells 21 are electrically connected with each other and a battery pack case 10 in which the battery module 20 is mounted. At the battery pack case 10 are formed a coolant inlet port and a coolant outlet port, which are disposed such that a coolant can flow from one side to the other side of the battery module 20 in the direction perpendicular to the stacking direction of the battery cells 21. At the battery pack case 10 are also formed inwardly-protruding beads 11, which are arranged in a concavo-convex structure and protrude inwardly of the inlet duct 12 such that the battery pack case 10 exhibits excellent durability or structural stability against an external force, such as twist or vibration. The external shape of the inwardly-protruding beads 11 is clearly shown in FIG. 2, which is a partial perspective view illustrating the external appearance of the battery pack case 10.
As shown in FIG. 2, the inwardly-protruding beads 11 are constructed in a concavo-convex structure having a large length (L) to width (W) ratio. The inwardly-protruding beads 11 are arranged parallel to each other.
Referring back to FIG. 1, small flow channels 22 are defined between the respective battery cells 21 of the battery module 20 such that a coolant can flow through the flow channels 22. Consequently, the coolant, introduced through the coolant inlet port 2, flows through the flow channels 22. At this time, heat generated from the battery cells 21 is removed by the coolant. After that, the coolant is discharged outside through the coolant outlet port 4.
However, the flow of the coolant, introduced through the coolant inlet port 2, is greatly disturbed by the inwardly-protruding beads 11 formed at the battery pack case 10 adjacent to the coolant inlet port 2, with the result that it is difficult to achieve uniform coolant flux distribution to the battery cells 21. Specifically, the width of the upper duct 12 is temporarily reduced at the positions where the inwardly-protruding beads 11 are located. As a result, the flux of the coolant flowing through the flow channels defined between the battery cells 21 located below the inwardly-protruding beads 11 is considerably reduced, and therefore, the coolant is driven to the front of the inwardly-protruding beads 11.
FIG. 3 is a graph illustrating the measurement results of coolant flux distribution between battery cells of the middle- or large-sized battery pack manufactured in the structure shown in FIG. 1. It can be seen from the graph that the mass flow rate X of the coolant between the battery cells at the positions b1, b2, b3, b4, and b5 where the inwardly-protruding beads 11 of the battery pack case 10 are located is considerably reduced with the increase in distance from the coolant inlet port.
Eventually, the coolant is not uniformly supplied to the respective battery cells 21, and therefore, the temperature difference between the battery cells 21 greatly increases. Such great temperature difference is one of the main causes greatly lowering the overall performance of the battery pack.
As a technology for solving a problem caused by the nonuniform distribution of a coolant, a technology for changing the flow direction of the coolant by a plurality of rectification plates installed in a coolant channel is disclosed in Japanese Patent Application Publication No. 2005-116342. However, this technology has a problem in that a process for installing the rectification plates is added to a conventional manufacturing process, and therefore, the manufacturing costs are increased. Furthermore, the disclosed technology has another problem in that the flow of the coolant in the battery pack is disturbed by the rectification plates, and therefore, the average remaining time, for which the coolant, introduced through the coolant inlet port, is discharged outside through the coolant outlet port, increases, whereby the cooling effect deteriorates.
Consequently, there is a high necessity for a technology to fundamentally solve the above-mentioned problems.